Current mirrors and their uses are well known. One representative text which discusses some of the conventional prior art relative to current mirrors is P. Horowitz and W. Hill, The Art of Electronics, Cambridge University Press, Cambridge, England (1980) at 71-74, which is hereby incorporated by reference. The simplest, or basic, current mirror is a two-transistor (bipolar or MOS) circuit, such as the example of FIG. 1. Although basic current mirrors of this type are useful in many situations, their output impedance is relatively low; this characteristic renders the simple current mirror unsuitable in many applications. Current mirrors having higher output impedances have been designed (such as the "Wilson" mirror circuit of FIG. 2 and the "cascode" current mirror circuit), but the increased output impedance has been achieved in these circuits at the expense of a reduction in output voltage compliance. That is, the output voltage of the Wilson and cascode mirror designs is confined to a smaller range than for the simple current mirror (as a percentage of supply voltage), and that output cannot approach the supply voltage as closely as can the output of the basic current mirror. Consequently, to achieve a similar output voltage swing the cascode and Wilson mirror circuits require a greater supply voltage than does the basic mirror circuit. The extra voltage drop added by the second transistor in the output circuit is particularly troublesome in MOS mirror circuits. The voltage drop across each transistor is large in such circuits, compared to comparable bipolar transistor circuits, and doubling it severely reduces the compliance of the output.
Accordingly, it is an object of the present invention to provide an improved current mirror circuit having higher output impedance than the basic two-transistor current mirror circuit.
Another object of the invention is to provide an improved current mirror circuit having, in addition to higher output impedance than a basic current mirror, greater output voltage compliance than that of a cascode or Wilson-type current mirror.